DIPTERA OF NORTH AMERICA. 3 



consider them as one, divided into several annuliform segments. 

 In the second section, the Brachycera, the two joints of the scapus 

 are likewise separated; the third joint, or first of the flagellum, 

 usually differs by its remarkably developed size and its anatomical 

 structure, causing it to be considered as a sensorial organ about 

 the nature of which entomologists are not yet agreed. The suc- 

 ceeding joints of the flagellum are much reduced in size, generally 

 very few in number, and often of unequal number in nearly related 

 genera, or even in species of the same genus. They even disappear 

 entirely in some genera (e. g., in Scenopimis). If they are extant, 

 they have usually the form of a style or bristle, the position of 

 which, according to its nature, is in fact apical, although, from 

 the development of the under side of the third joint, the bristle 

 often seems inserted on its back, or even, in some instances, in 

 the immediate vicinity of the base itself. In the genera, in which 

 the first joint of the flagellum is not of a remarkable size, the 

 following joints are generally more numerous, and either all or the 

 greater part of them share the peculiar organization showing their 

 function to be that of a sensorial organ. They are applied at the 

 same time so closely to the first joint of the flagellum, that we are 

 compelled to consider all of them together as one, divided into 

 several segments, or the terminal one as a style or bristle of a single 

 joint, formed by the other joints of the flagellum. Consequently 

 the essential difference between the sections Nemocera et Brachycera 

 is this, that in the latter the number of joints of the flagellum is 

 not only smaller, but also that the lower joint, sometimes a few 

 joints, always the lower ones, rarely all, have a more distinct de- 

 velopment, and at the same time a peculiar anatomical structure 

 undoubtedly proving their function to be that of a sensorial organ. 

 It cannot be denied that those families of Brachycera in which 

 several of the joints of the flagellum are so soldered together as 

 to form one compound and annulated mass, stand nearest to the 

 section of Nemocera, and that amongst these families the Xylopha- 

 yidce must be placed immediately on the limit of both sections. It 

 is more difficult to point out a family of Nemocera, which comes 

 nearer to the section of Brachycera than all the others ; in general 

 the families of Rhyphidce and Bibionidce may be considered as those 

 to which this station must be assigned. It is a fact that some dis- 

 coveries made in modern times have obliterated to a certain degree 

 the sharpness of the limit which was considered to exist between 



